Voltage-controlled oscillators (VCO) are critical components in phase-locked loop (PLL) circuits used to synthesize radio frequencies in communication transceivers. A typical PLL circuit is shown in FIG. 1. The PLL implements a feedback system and consists of a VCO, N counter, phase/frequency detector (P/FD) and charge pump (CP). The feedback drives the phase difference (Δθ) between the reference signal (Ref) and the output of the N counter towards zero by creating a control voltage vctrl that adjusts the VCO to the proper frequency and phase offset. When locked, the PLL generates an output signal (RF signal) with a frequency fVCO equal to;fVCO=NfREF where N is the value of the N-counter and fREF is the frequency of the reference signal.
The PLL and associated VCO typically support one or more frequency bands. A single frequency band, such as those used in the Personal Communication System (PCS) or the Universal Mobile Telephony System (UMTS), spans 60 MHz, while these two bands together cover 180 MHz. However, using a conventional VCO to cover such a wide frequency band or multiple bands produces degraded performance and phase noise. This makes the PLL design especially challenging.
Therefore, it would be advantageous to have a VCO that can operator over a wide frequency range without sacrificing performance or degraded phase noise.